What do you think about this :
Each time you beat a crature, a part of it\'s soul is \"stocked\" in a gem/crystal. When it\'s enough loaded, you can summon the creature (on your side), but only once, spending the creature\'s soul.
There would be different gems/crystals with different properties, different prices, different loading capacity, different number of \"slots\" (a gem/crystal with 2 slots acts like 2 gems/crystals of this type, but only takes 1 space in inventory), ...